Mobile communication systems, such as fourth generation (4G) cell based mobile communication system such as LTE (Advanced) systems (Long Term Evolution, which is the most recent step forward from cellular 3G services) or the Wideband Code Division Multiple Access (WCDMA) system, comprise base-stations which provide a wireless or ‘air interface’ between the mobile phone or other mobile equipment and the base-station.
A base station for a mobile communication network may comprise different types of units including one or more radio equipment units coupled to respective antennas and one or more radio equipment controller units for controlling the radio equipment unit. The radio equipment unit basically performs the radio frequency, RF, related functions such as frequency shifting, sampling of the RF signals, quantization of I and Q values etc. The radio equipment controller performs baseband functions and controls the radio equipment unit.
In the past, base-stations would be provided with a small cabinet at the base of the antenna tower, in which appropriate equipment to perform all base-station functions was provided. Thus each antenna would have its own dedicated equipment for both the RF functions and the baseband functions. However, newer generations of radio base station systems use a split into sub-systems so that the baseband functions are separated from the RF functions and provided in another subsystem.
The baseband subsystem is typically in a cabinet or other housing, and connected to a remote RF subsystem via an optical fiber of suitable length or other, e.g. wired, link suitable to transport signals over a long distance (typically the baseband and RF subsystem are separated by a distance of 1 km or more, e.g >10 km). The remote RF subsystem is housed in what is commonly referred to as a “radio head” which further contains the antenna. Furthermore, the baseband subsystem may be connected to several antennas separated from each other by relatively long distances (>1 km for instance) and control the RF subsystems of multiple radio units. The radio-heads of several spaced-apart antennas are thus simultaneously controlled by the same base-station, located at a distance (e.g. >1 km, such as >10 km) from the radio-heads.
In such new generations of base-stations, multiple radio equipment controller units and/or radio equipment units may be coupled in a chain, while such a chained unit may process part of the data samples and/or control data and forward a further part to a subsequent unit. The units may have, for interfacing between the units, a data interface for streaming data samples using a transmit clock. The transmit clock of the chain typically needs to be synchronized with high precision to an external synchronization signal, such as a GPS signal or a signal generated with e.g. the precision time protocol (as standardized in IEEE 1588). For example both the Open Base Station Architecture Initiative (OBSAI) and the Common Public Radio Interface (CPRI) require such high accuracy synchronization.
In the known solutions, synchronization to the external synchronization signal requires a separate device, e.g. an external integrated circuit which comprises a phase locket loop (PLL) driven by e.g. a crystal oscillator and a synchronization device which synchronizes the PLL to the external synchronization signal or comprising an external controllable oscillator which is controlled by the REC subsystem to be synchronized to the external synchronization signal. United States patent application US2009/0238154 for example describes synchronization of assemblies in a base station to a reference clock signal. A local clock signal and a frame are formed in a first assembly. The clock signal and the frame are transmitted, using a synchronous transmission with a predictable propagation time, to a second assembly. A reference clock signal is received in the second assembly, and a phase difference and a time difference between the transmitted clock signal on the one hand and the reference clock signal on the other hand are determined. The phase difference and the time difference are transmitted from the second assembly to the first assembly via a link without a predictable propagation time. The phase difference and the time difference are used in the first assembly to determine a manipulated variable which controls the formation of the local clock signal, such that the first and the second assemblies are synchronized in time. However, the transmission from the second assembly to the first assembly introduces latency between the measurement and actual control of the local clock signal in the first assembly.
However, such a separate synchronization device may not be accurate enough. CPRI the synchronization between radio equipment controller (REC) and radio equipment (RE) requires an accuracy of 1 nSec and current synchronization solutions, which are based on external controllable oscillators, may not be accurate enough to comply with the requirements of the CPRI specification.
However, such a separate synchronization device may not be accurate enough. CPRI the synchronization between REC and RE requires an accuracy of 1 nSec and current synchronization solutions, which are based on external controllable oscillators, may not be accurate enough to comply with the requirements of the CPRI specification.